Insulation systems having improved partial discharge resistance, and method for producing same

ABSTRACT

Disclosed are insulating electric conductors against partial discharge and a method for producing an insulation system having improved partial discharge resistance and such an insulation system. There is an erosion-inhibiting effect of adhesion promoters, such as organic silicon compounds, added to resin when admixing nano particulate fillers. Good results may be attributable to a type of particle wetting of the nano particles as a result of particle wetting with organosilanes. The admixture of adhesion promoters with the resin before the addition of the nano particulate filler provides considerable advantages.

The present invention pertains generally to the field of the insulationof electrical conductors against partial discharge, and specifically toa method for producing an insulating system having improved partialdischarge resistance and to an insulating system having improved partialdischarge resistance.

In rotating electrical machines, such as motors or generators, thereliability of the insulating system is critically responsible for theiroperational integrity. The insulating system has the function ofelectrically insulating electrical conductors (wires, coils, bars)durably from one another and from the laminated stator core or theenvironment. Within high-voltage insulation, distinctions are made ininsulation between partial conductors (partial conductor insulation),between the conductors or windings (conductor or winding insulation),and between conductor and ground potential in the slot and winding headregion (main insulation). The thickness of the main insulation isadapted both to the nominal voltage of the machine and to theoperational and fabrication conditions. The competitiveness of futureplants for energy production, their distribution and utilization, iscritically dependent on the materials employed and technologies appliedfor the insulation.

The fundamental problem with insulators loaded electrically in this waylies in the so-called partial discharge-induced erosion with formationof what are called “treeing” channels, which lead ultimately to theelectrical breakdown of the insulator.

High-voltage and medium-voltage machines currently employ what arecalled impregnated laminar mica insulation systems. In these systems,the form-wound coils and conductors produced from the insulated partialconductors are enwound with mica tapes and impregnated with syntheticresin preferably in a vacuum pressure impregnation (VPI) process. Thecombination of impregnating resin and the carrier tape of the micaprovides the present mechanical strength and also the required partialdischarge resistance of the electrical insulation.

Mica paper is converted, in line with the requirements of the electricalindustry, into a more stable mica tape. This is done by bonding the micapaper to a carrier material having a high mechanical strength, by meansof an adhesive. A feature of the adhesive is preferably that it has ahigh strength at room temperature, in order to ensure the join betweenmica and carrier, and passes into a liquid state at elevatedtemperatures (60° C.-150° C.). This allows it to be applied as anadhesive at elevated temperature in liquid form or in a mixture with avolatile solvent. After cooling has taken place or the solvent has beenremoved, the adhesive is present in a solid yet flexible form, andallows the mica tape to be applied, for example, around Roebel barsconsisting of partial conductors and form-wound coils at roomtemperature, with the adhesive properties of the adhesive preventingdelaminations of the mica paper from the carrier material. The resultingmica tape is wound in a plurality of plies around electrical conductors.

High-voltage and medium-voltage motors and generators employ laminarmica insulation systems. In these systems, the form-wound coils producedfrom the insulated partial conductors are enwound with mica tapes andimpregnated with synthetic resin primarily in a vacuum pressureimpregnation (VPI) process. In this case, mica is used in the form ofmica paper, and as part of the impregnation the cavities located betweenthe individual particles in the mica paper are filled with resin. Thecombination of impregnating resin and carrier material of the micaprovides the mechanical strength of the insulation. The electricalstrength comes about from the multiplicity of solid-solid interfaces inthe mica used. The resulting layering of organic and inorganic materialsforms microscopic interfaces whose resistance to partial discharge andthermal stresses is determined by the properties of the mica platelets.As a result of the complicated VPI process, even very small cavities inthe insulation must be fully filled with resin, in order to minimize thenumber of internal gas-solid interfaces.

For the additional improvement of the resistance, the use ofnanoparticulate fillers is described.

The combination of impregnating resin and the carrier tape of the micaprovides the present mechanical strength and also the required partialdischarge resistance of the electrical insulation.

As well as the VPI process, there is also the Resin Rich technology forproducing and impregnating the mica tape, in other words the insulatingtape and then, subsequently, the insulating system.

The main difference between these two technologies is the constructionand manufacture of the actual insulating system of the coils. Whereasthe VPI system is complete only after the impregnation and after thecuring of the winding in a forced air oven, the leg of the Resin Richcoil, cured separately under temperature and pressure, constitutes afunctioning and testable insulating system even before installation intothe stator.

The VPI process operates with porous tapes, forming a solid andcontinuous insulating system under vacuum with subsequent exposure ofthe impregnating vessel to overpressure after curing in the forced airoven.

In contrast to this, the manufacture of Resin Rich coils is morecomplex, since each coil leg or winding bar has to be manufacturedindividually in specific baking presses, leading to a specific increasein the costs of the individual coil.

In this context, mica tapes are employed that are impregnated with apolymeric insulating substance which is present at what is called aB-stage. This means that the polymer, usually aromatic epoxy resins(BADGE, BFDGE, epoxidized phenol novolaks, epoxidized cresol novolaks,and anhydrides or amines as hardeners), is partially crosslinked and isthus in a tack-free state, but on further heating is able to melt againand be ultimately cured, so as to be brought into the final shape. Sincethe resin is introduced in an excess, it is able, during the finalpressing operation, to flow into all cavities and voids, in order toattain the corresponding quality of insulation. Excess resin is pressedout of the system by the pressing operation.

From the literature it is known that the use of nanoparticulate fillersin polymeric insulating substances leads to significant improvements inthe insulation in respect of the electrical longevity.

A disadvantage of the known systems, especially of those based on epoxyresins, is the rapid degradation of the polymeric matrix on exposure topartial discharge, here referred to as erosion. Implementing the polymermatrix with erosion-resistant nanoparticles (aluminum oxide, silicondioxide) causes its exposure, brought about by incipient breakdown ofthe polymer, referred to as polymer degradation.

It is the object of the present invention to enable an insulating systemhaving improved partial discharge resistance.

Presented in accordance with one aspect of the invention is a method forproducing an insulating system having improved partial dischargeresistance, comprising the following method steps:

-   -   providing an insulating tape which comprises a mica paper and a        carrier material, which are bonded to one another by means of an        adhesive,    -   enwinding an electrical conductor with the insulating tape, and    -   impregnating the insulating tape wound around the conductor with        synthetic resin, characterized in that an adhesion promoter is        added to the synthetic resin system before the nanoparticulate        filler is added.

According to a further aspect of the invention, an insulating systemhaving improved partial discharge resistance is presented, having aninsulating tape which is wound around an electrical conductor andcomprises a mica tape joined to a carrier material, the insulating tapebeing impregnated with a synthetic resin, characterized in that theimpregnated insulating tape is interspersed with a nanoparticulatefiller which is agglomerated at least partly via an adhesion promoter.

It is known that in contrast to polymeric insulating substance,inorganic particles are not destroyed or damaged, or only to a verylimited extent, on exposure to partial discharge. The resultant erosioninhibition effect of the inorganic particles here is dependent onfactors including the particle diameter and the particle surface whichgenerates from it. It is found here that the greater the specificsurface area of the particles, the greater the erosion inhibition effecton the particles. Inorganic nanoparticles have very high specificsurface areas, at 50 g/m2 or more.

Generally speaking, an unfilled or mica-based insulating substance basedon epoxy resins exhibits rapid degradation of the polymeric matrix onexposure to partial discharge. Implementing the polymer matrix witherosion-resistant, nanoparticulate filler (aluminum oxide, silicondioxide) results in exposure of the nanoparticulate filler, caused bypolymer degradation.

As the duration of erosion increases, a firmly adhering, sheetlike layeris gradually formed on the surface of the test element, consisting ofexposed nanoparticulate filler. As a result of this particlecrosslinking of the nanoparticulate filler, caused by the erodedpolymer, the surface is passivated and the polymer beneath thepassivation coat is effectively protected from further erosion underpartial discharge exposure.

Surprisingly it has been found that through the use of adhesionpromoters, more particularly of silanes, in the impregnating resinand/or in the Resin Rich resin, it has been possible to inhibit erosion.

Adhesion promoters are usually organosilicon compounds which throughcondensation reactions are attached chemically to the surface of fillersor nanoparticles. The adhesion promoter gives rise to improvedattachment of the particles to the polymer matrix, thereby producing animproved erosion resistance. This is directly dependent on the surfacearea of the filler, which is why the use of adhesion promoters onparticles with small diameters improves the erosion resistance to aparticular degree. A coating of this kind corresponds to the first layerin the Multi Core model of Prof. Tanaka in Tanaka et al., Dependence ofPD Erosion Depth on the Size of Silica Fillers; Takahiro Imai*, FumioSawa, Tamon Ozaki, Toshio Shimizu, Ryouichi Kido, Masahiro Kozako andToshikatsu Tanaka; Evaluation of Insulation Properties of Epoxy Resinwith Nano-scale Silica Particles, Toshiba Research Cooperation.

It has been shown that the use of organosilanes can be utilizedsynergistically with nanoparticles by admixing adhesion promoters suchas silanes to the impregnating resin or Resin Rich resin.

One particularly advantageous embodiment of the invention lies in thesynergistic utilization of the described model of the passivation coatunder PD loading, and the improvement in erosion inhibition through theuse of organosilanes in mica-based high-voltage insulating systems. Thisis achieved by the added organosilanes exerting a positive influence onthe formation and mode of action of the passivation coat that formsunder PD loading. The enhanced erosion resistance can be explained byspontaneous sintering of the particles, catalyzed by the use oforganosilanes, and the formation of a quasiceramic layer. The use oforganosilanes here is not confined to their use for the coating ofnanoparticles, but may also take place, as described for the first timehere, by their direct addition as a component to the reactive resinformulation.

Elucidated below are possible fundamental principles for advantageouslyimproved erosion resistance through the use of organic silanes in theresin formulation:

Organic silanes are activated under PD loading and lead, by means ofcondensation reactions, for example, to crosslinking of thenanoparticles via siloxane bonds which form.

POSS (polyhedral oligomeric silsesquioxanes) constitute the smallestpossible unit of nanoparticulate organic silanes and allow thecrosslinking of nanoparticles under the influence of PD energies.

Organic silanes (mono- or polyfunctional), with their reactive groups,allow the crosslinking of nanoparticles through chemical reactions withreactive groups on the nanoparticle surface.

In accordance with the invention, particularly advantageous embodimentsresult with reactive resin formulations constructed from the followingcomponents:

The resin basis is formed, for example, by an epoxy resin and/or apolyurethane resin.

The hardener comprises an anhydride, an aromatic amine and/or analiphatic amine, for example, as functional group.

The nanoparticulate filler has a particle size of, for example, between2.5 to 70 nm, more particularly from 5 to 50 nm in a concentration ofbetween 5 and 70 wt %, more particularly between 10-50 wt % on the basisof SiO₂ or Al₂O₃. Further fillers, additives, and pigments may bepresent.

The adhesion promoter is preferably an organic silicon compound, such asan organosilane and/or POSS. They are present in the syntheticresin—again preferably—at a concentration of between 0.1 and 45 wt %,more particularly of 1-25 wt %.

The use of adhesion promoters such as organic silicon compounds as partof the resin formulation in combination with the stated componentsoffers the following advantage—that the use of adhesion promoter, namelysilane as part of the reactive resin is possible in higherconcentrations than when using silanes as adhesion promoters of theparticles before the addition to the reactive resin. Through the use ofthe organosilane as part of the resin formulation it is possible,moreover, to use a substantially greater number of silanes, since thespectrum of organic silanes that can be used is increased if they do nothave to be anchored in the form of coatings to the surfaces of theparticles.

As a result of the advantages elucidated, the spectrum of organosilanesthat can be used is very wide. Typically, silanes are used which containone or more functional groups having sufficient reactivity to be able toundergo reaction with the particle surface. The silanes used may have 1to 4 functional groups.

FIG. 1 shows schematically a general mechanism for in situ particlecrosslinking, using a difunctional organosilane as an example.Fundamentally, silanes may possess one to four reactive functionalgroups, in order to exert a positive effect on the erosion resistance.These functional groups have the property of being able to react withthe particle surface, resulting in the large spectrum of organosilanes.

The mechanism of particle crosslinking proposed in FIG. 1 with adifunctional silane; R₁=hydroxyl, alkoxy, halogen, glycidyloxy;R₂=alkyl, glycidyloxy, vinyl, propylsuccinic anhydride,methacryloyloxypropyl shows the substitution of the radicals R₁ on thesilane by nanoparticles. R₂ also be amidic, sulfidic, oxidic, or H.“Amidic, oxidic, and sulfidic” here means that further organic R′₂ maybe present, bonded to the silicon via nitrogen, oxygen, or sulfur.

The particles 1 and 2 are both bonded to the silicon core 3 bysubstitution of the radicals R₂ on said core 3, with an increase intemperature, for example, and are therefore located in the immediatevicinity of one another, and are crosslinked via the silicon core 3.

The potential of nanotechnology is evident here again when usingnanoparticulate fillers in combination with the silanes of theinvention, as for example in the presently employed insulating materialsbased on mica.

In FIGS. 2 to 4, reference samples which are experimental specimens(represented by interrupted lines) are contrasted in each case withembodiments of the invention. The experimental specimens correspond inreduced-size form to the state of the art in respect of insulated Cuconductors in stators of hydroelectric generators or turbogenerators.They are measured under electrical field loading to the point ofelectrical breakdown. Since the electrical strength of the insulatingsystem under operational exposure runs to several decades, theelectrical durability tests take place with multiply overdimensionedelectrical field strengths.

The graph shown in FIG. 2 represents the average values for theelectrical lifetime of batches of seven test specimens under threedifferent field exposures for both a standard insulating system (mica)and a nanoparticulate/silane filled insulating system. The unfilledsystems (designated Micalastic) have a fraction of about 50 wt % micaand 50 wt % resin. The stated fraction of nanoparticles reduces thefraction of resin correspondingly. The fraction of mica remains constantin each case.

The lifetime curves shown in FIG. 2 for unfilled andnanoparticulate-filled high-voltage insulating systems (Micalastic(black) and, Micalastic with nanoparticles 10 wt % (diameter about 20nm) and organic silane (3-glycidyloxypropyltrimethoxysilane, 5 wt %)show clearly that the latter systems exhibit a significantly extendedlifetime under given loading.

FIG. 3 shows corresponding lifetime curves for unfilled andnanoparticulate-filled high-voltage insulating systems (Micalastic(black) and, Micalastic with nanoparticles 10 wt % (diameter about 20nm), octamethyltrisiloxane 2.5 wt %. Here again, the virtually parallelshift in lifetimes toward longer times is readily apparent.

FIG. 4, finally, shows the lifetime curves for unfilled andnanoparticulate-filled high-voltage insulating systems (Micalastic(black) and, Micalastic with nanoparticles 10 wt % (diameter about 20nm), POSS (2.5 wt %).

Comparing the lifetime of each of the groups, it is found thatimprovements in the lifetime in the factor of 20 to 30 are achieved.Both lifetime profiles have the same slope, and so it appears possibleto transpose the prolonged lifetime directly to operational conditions.

Insulating systems with a nanoparticulate fraction of up to 35 wt % arepossible.

The invention shows for the first time the surprising erosion-inhibitingeffect of adhesion promoters such as organic silicon compounds, presentin the resin, when nanoparticulate filler is added. The introduction ofthe adhesion promoter into the resin before the nanoparticulate fillerresults in surprisingly good outcomes. There is discussion as to whetherthe good results as illustrated in FIGS. 2 to 4 are attributable to akind of particle crosslinking of the nanoparticles by particlecrosslinking with the organosilanes. At any rate it is possible to showimpressively that the admixing of adhesion promoters to the resin priorto the addition of the nanoparticulate filler is able to bringconsiderable advantages.

1. A method for producing an insulating system having improved partialdischarge resistance presented, comprising the steps: providing aninsulating tape which comprises a mica paper and a carrier material,which are bonded to one another by means of an adhesive, enwinding anelectrical conductor with the insulating tape, producing synthetic resinby introducing a resin system with an adhesion promoter, into which ananoparticulate filler is incorporated, and impregnating the insulatingtape wound around the conductor with the synthetic resin.
 2. The methodas claimed in claim 1, wherein the resin system has a resin basisselected from the group consisting of epoxide-based resins and/orpolyurethanes.
 3. The method as claimed in claim 2, further comprisingusing an organosilicon compound as an adhesion promoter for theadhesive.
 4. The method as claimed in claim 2, wherein thenanoparticulate filler is selected from the group consisting of metaloxides, metal nitrides, metal sulfides and/or metal carbides.
 5. Aninsulating system with improved partial discharge resistance,comprising: an insulating tape which is wound around an electricalconductor, the insulating tape comprises a mica tape joined to a carriermaterial, and the insulating tape is impregnated with a synthetic resin,wherein the impregnated insulating tape is interspersed with ananoparticulate filler which is crosslinked at least partly via anadhesion promoter.
 6. The insulating system as claimed in claim 5,wherein the nanoparticulate filler is present in a particle size of 2.5to 70 nm.
 7. The insulating system as claimed in claim 5, wherein thenanoparticulate filler is present in the synthetic resin in aconcentration of between 5 and 70 wt %.
 8. The insulating system asclaimed in claim 5, wherein the adhesion promoter is an organic siliconcompound.
 9. The insulating system as claimed in claim 5, wherein theadhesion promoter is present in a concentration of 0.1 to 45 wt % in thesynthetic resin.
 10. The method as claimed in claim 1, furthercomprising using an organosilicon compound as an adhesion promoter forthe adhesive.
 11. The method of claim 1, further comprising in producingthe synthetic resin, mixing the resin with an adhesion promoter andafterward incorporating the nanoparticulate filler in the resin.